Add "Expand whole file" option/button for each file in PR #2504

Currently, if I want to understand the context in which change occurs I have several options, all of which have their own issues:

• I need to click Expand Up/Expand Down buttons

  <img width="108" alt="Screen Shot 2019-10-26 at 9 15 14 AM" src="https://user-images.githubusercontent.com/1374723/67615151-273c4e80-f7d1-11e9-9d2a-2f584cceb8c1.png"> Problems: If relevant lines are too far away (like 100 lines above/below), I am doomed to click those for all eternity (it adds only 20 more lines per click)

• I can open the file in another tab ... -> View file Problems: I cannot use browser search to look for matches across files now, or I have to switch back-and-forth between tabs. And I also lose any PR comments in the file I opened this way.

I would like each file in PR to have a button which will expand the whole file and allow me to do browse text searches on all context without compromises.

One usability suggestion I would add - maybe if the file is not that large, then clicking Expand Up/Expand Down buttons would:

• on first click - works are normal
• on second click - expands the whole file (we can change icon/tooltip to reflect this)

This will avoid cluttering the interface with new buttons while keeping the intent.

Example URL: https://github.com/sindresorhus/refined-github/pull/NNNN/files