[web] Introduce markdown component for chat mentions

Summary:
Solution for ENG-4560.
This diff creates a new component for chat mentions which handles bolding mentions when user has access to given chat.

Depends on D8851.

Test Plan:
Before testing, please apply the entire web stack.

1. Create a community with three users.
2. Create a private chat for two chosen users.
3. Mention private chat in community chat (e. g. @[[256|8282:default text]]).

For users assigned to previously created private chat, the mention should be bold and render current thread name:

For users who are not in private chat, the mention should be rendered with plain text message style:

Reviewers: tomek, inka, rohan

Reviewed By: tomek, inka, rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8852