[web] Display pinned messages not present in Redux

Summary:
As mentioned in ENG-3924, there is an issue with some pinned messages where if they are
not present in Redux, they will not show up in the modal.

This is because messageListData will incorrectly set the messageInfo's
isPinned flag to false, and therefore we will filter it out. This diff
resolves this on web by keeping a Set of pinned messgae IDs, and
filtering out messages that are not part of the set, rather than based on
their isPinned value. This maintains the same behaviour for pinned
messages that were fetched, but also covers the case where they are not
part of Redux.

native will come in the next diff

Test Plan:
Please see the before and after for a pinned message that had
no reactions/related messages and was not present in Redux. Confirmed
separately that pinned messages on web still behave as expected.

Before:

After:

Reviewers: atul, inka, ginsu

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8096