ui: Read source UI improvements

[manojVivek]

Fixes:

• Eliminate redundant source requests for the same build id.
• Clicking on a line number should highlight that line and persist it in the URL so that I can send the URL to someone, and it will scroll to that position
• It seems that somehow the hover-bubble keeps re-rendering or something else that makes it feel laggy when having sources open
• Horizontal scrollbar for the source viewer.
• Fallbacks handled when the line number field is missing.

[alwaysmeticulous]

🤖 Meticulous spotted visual differences in 33 of 303 screens tested: view and approve differences detected.

_{Last updated for commit 394a58b. This comment will update as new commits are pushed.}

[yomete]

lgtm!

[brancz]

Amazing work!