[web] Keyboard support for typeahead [6/13] - Get rid of entire text and rename matched strings

Summary:
Some renaming of arguments and variables.
Removed entireText from TypeaheadMatchedStrings as we only needed its length in getTypeaheadTooltipActions function.
Length of entire text can be easily obtained by textBeforeAtSymbol.length + usernamePrefix.length + 1 where 1 stands for @ symbol.

Test Plan:
Typeahead without keyboard support works.
Final tests performed in last diffs.

Reviewers: inka, kamil, ginsu, tomek

Reviewed By: inka, tomek

Subscribers: ashoat, atul, tomek, ginsu, kamil, inka

Differential Revision: https://phab.comm.dev/D6046