Unify sidebar buttons to use the same image

[GuillaumeGomez]

Part of #139832.

The source sidebar looks like this with the new image:

r? @notriddle

[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha

[notriddle]

Is there a particular reason for using the hamburger icon in desktop mode, instead of ? When the file icon was originally chosen in #119066, the main reason was that hamburger icons usually open modals, but, in desktop mode, this button doesn't.

In any case, this is my only question, and it's not that big of a deal. We finally have empirical feedback to work with here, and that trumps any amount of theorizing.

@rfcbot fcp merge

[notriddle]

@rfcbot fcp cancel

Uuuugggghhhh... why do I always forget to check the list of T- tags before calling fcp on a gui test like this?

[rfcbot]

@notriddle proposal cancelled.

[notriddle]

@rfcbot fcp merge

[rfcbot]

Team member @notriddle has proposed to merge this. The next step is review by the rest of the tagged team members:

• @GuillaumeGomez
• @camelid
• @jsha
• @notriddle

Concerns:

• hamburger-button-on-desktop (Unify sidebar buttons to use the same image #140135 (comment))

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[notriddle]

@rfcbot concern hamburger-button-on-desktop

#140135 (comment)