Add User Timing API marks and measures to Project Overview page / repository browser

Proposal

Alike what we did for the MR Changes tab, we should add User Timing API's marks and measures to:

• Project Overview
• Repository browser pages

Events/moments

What are the moments we should mark? (open for discussion)

Keywords	Description
project_overview_header	The header of the project, with title, description, info on commits, etc.
repository_first_files_shown	As soon as the filenames show up for the first batch
repository_first_last_commit_shown	As soon as the first last_commit information (in the table) show for the first batch
repository_files_start	When we trigger the app
repository_files_end	When the files stop fetching/rendering (including when truncated at 1000/limit)
repository_last_commit_folder_shown	When the last commit (commit well) is shown
...	...

Example

This is how these displays in the Site Speed reports, under the Timings section.

This is how it displays the console when Performance Bar is enabled: