User mapping - Handle mentions and participants in GitHub importer

Similar to #477097 (closed)

Problem

At the moment, importers do not handle mentions very well, and some importers do not handle them at all, causing incorrect users to be mentioned and added as participants, which causes a lot of confusion.

With the introduction of improved user mapping, handling them will be more challenging, as we would need to rename mentions to placeholder users and then, after the reassignment, replace the placeholder user mentions with the actual user. It will be even more difficult if the placeholder limit is reached and the source user is mapped to the unique ImportUser.

This issue proposes disabling mentions for imported context, which would greatly simplify things. The same solution could be easily implemented in all importers.

Proposed solution

Wrap usernames in backticks

Same as in User mapping - Handle mentions and participants... (#477097 - closed).

Let's wrap usernames mentioned in notes or description text in backticks during the import to prevent links from being added to the usernames.

See an example below:

Since usernames would be wrapped in backticks, I believe, we wouldn't need to change how the participant's lists are built.

The advantage is that users would be able to edit the contribution and mention users.