Change Issue activity to match MRs

What does this MR do and why?

(see relevant discussion in Slack or transcript of it in comments below)

This changes all actions in an issue except for closing to use the small dot style used on MRs. It also changes the size of the icons in an issue's activity feed to match those in MRs.

This MR also removes some custom styling that was applied in merge_requests.scss and upstreams it into issuables.scss.

Note that this is changing a very central piece of functionality -- careful and extra UX review will be prudent here.

Screenshots or screen recordings

Scenario	Before	After	What's different?
Issue with activity			small dots are used for all issue activity EXCEPT for closure. When an issue is closed, the same icon and color is used as previously, but the size and alignment are changed to match the size and alignment of icons in the Activity timeline of an MR.
MR with approval(s)			No change
MR with generic activity			No change
MR that has been closed			No change
MR that has been merged			No change

How to set up and validate locally

1. Check out this branch locally in the GDK
2. navigate to an issue that has activity

• you will see that all items in the activity feed except for issue closure items use the same small dot as MR activity feed items
• activity feed items representing an issue closure will use the same icon and color as before, but will be smaller, using the same size and alignment as the icons used in the activity feed for MRs

3. navigate to one or more MRs that include closure activity, merging, and approving

• you should see that there are no visual differences to current master branch on MR activity feeds

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

✂️ UX Paper Cuts 16.6 → Activity (#425468 - closed)