Add find in page to talk pages

[staykids]

Phabricator: https://phabricator.wikimedia.org/T319470 (partially)

Notes

Adds find in page to talk pages.

More refinement work in a forthcoming PR remains, including:

• More precise scrolling to active search results when in a long blob of text
• Dismissing the find in page bar when the user performs a competing action (like tapping to post a reply)

Test Steps

1. Open a talk page and tap the find in page tab bar item
2. Try searching for a term, then tapping the keyboard bar actions (next, previous, clear, etc.)
3. Change themes and try again, confirming there are no render issues

[tonisevener]

This is great to see! Nice job working with the existing search view.

I have a few actual code review comments, and then the rest are suggestions I tinkered with for getting the scrolling to work a bit more predictably. Let me know what you think!

[staykids]

@tonisevener Thanks for the great suggestions - updated and ready for another look!

[tonisevener]

Looking good! I had to adjust a little bit from some merge conflicts from main. We are now letting talk page items display that have no replies, so that caused some adjustments in the view models. main is also utilizing the removingInitialNewlineCharacters method in more places, which threw off some of the ranges for the active highlight.

I patched these up as best I could, but it could probably use more attention with the next PR. I'm going to go ahead and merge to get the current functionality in though.

I also noticed that it's no longer sluggish when I type, so perhaps the move away from reloadData improved that.