[2.3] Fix last modified dates in settings and lexicon grids + lexicon grid / filter combos improvements

This PR would solve https://github.com/modxcms/revolution/issues/11762

It adds a new lexicon string not_modified (only for English, the translations go over crowdin now right?) which is displayed if there is no valid timestamp (or 0) from strtotime().

Additionally the last modified dates in settings grids are respecting the manager_date_format and manager_time_format... as these are specified to be compatible with php's date() function I had to switch strftime() with date() (to avoid another date format system setting), which causes the loss of localized dates (at least when using month names and other strings) which (as far as I know) only can be provided trough strftime().

**To solve this problem I'll look into transforming the pure timestamps with extjs when the grid loads.**

When working on this, I stumbled over the lexicons grid and removed the combobox paging for namespaces, topics and languages as discussed here https://github.com/modxcms/revolution/issues/11713 and made the comboboxes typeahead instead. I'm not sure if this is too radical or if for this particular grid this is acceptable. I cannot really imagine people having thousands of namespaces or topics which would justify paging inside the combobox? Does anybody know of such a project? For the languages combo it's not even worth the discussion as there is a quite limited number of languages I think...

Tested the following grids:

- System Settings
- User Group Settings
- User Settings
- Context Settings
- Lexicon Entries