[web] Change names from Mention/Mention suggestion to Typeahead

Summary:
Little followup on typeahead.
We had those inconsistent names in file concerning typeahead, as early on I used name "mentioning" for the feature.
Typeahead feels more natural and more people use that name, so I renamed files and functions

Test Plan: Tested if everything still works properly.

Reviewers: tomek, kamil, atul

Reviewed By: tomek

Subscribers: ashoat, atul, kamil, tomek

Differential Revision: https://phab.comm.dev/D5757