The aim is to be able to use separate delegate for things like codeblocks and quotes so that they can be styled differently.

Refactor `LinkPreviewer` to take an event and put the functions for getting the link in the class itself. This means the functions in `EventHandler` are no longer required.
This mr also sets up `LinkPreviewer` so that it is automatically updated when an event is edited. This includes changing the link if edited, and it can handle a message having a previous link removed or a one added when one didn't exist before.
Also adds test suite.
Make sure that custom emojis in inline code blocks are not turned into images.
By calling preprocess text in `texthandler` the whole function can be simplified as it will now never be called on any text inside any code block (which was the reason for all the split stuff previously).
BUG: 477512
Similar to text handler, pull out the disparate array of functions which format information from an event ready for display in the UI and put in a handler class with a test suite.
requires https://github.com/quotient-im/libQuotient/pull/686
This move the finding of links and the creation of a `linkpreviewer` into c++.
- The links are now extracted from the text in `texthandler`
- The `messageeventmodel` now creates and stores `linkpreviewers` for events that have links in the current room.
Two new model roles have been created to let a text delegate know when the link preview should be shown (`showLinkPreview`) and pass the link previewer (`linkPreviewer`). Empty link previewer are returned where link don't exist so the qml doesn't have to have checks for whether the parameters are undefined.
Improve the handling of text both when sending and receiving.
The main feature is to fix the linked bug (and a host of others that are unreported but similar) which is caused by the fact that we don't properly clean html. This mr does that as per the matrix spec https://spec.matrix.org/v1.5/client-server-api/#mroommessage-msgtypes. So any disallowed tags or attributes are removed and it does the special handling for certain attributes.
Additionally the functions are also designed to cover any other text formatting required, particularly fro received strings.
The receive side is covered by 2 functions `handleRecieveRichText` and `handleRecievePlainText`. The rich/plain in the function name refers to the output type not the input type (both can take plain and rich input), so `handleRecieveRichText` is called to get a string suitable to go in a rich text control and `handleRecievePlainText` for a plain control.
The functions also handle the following some of which was previously handled by `eventToString` in `NeoChatRoom`:
- Strip and reply from the string
- Format any user mentions
- Linkify links in plain strings
- Handle mxc urls in rich text (uses the new `room->makeMediaUrl` functionality from libQuotient)
- `handleRecievePlainText` also deals with markup making `NeoChatRoom->subtitle` redundant
There is also an extensive test suite which defines the behaviour and the best way to review this is probably to look at the tests and decide whether you agree with the expected output given the inputs and/or if there is any missing behaviour.
The final aim especially with the test suite is to give us a framework to make further updates in the future easier and hopefully prevent a new feature breaking old behaviour with the tests.
BUG: 463932 \
BUG: 466330 \
BUG: 466930