Make hover/focus/active states consistent for dropdowns

Mentioned in issue gitlab-foss#24063 (moved)

In order to consolidate all of our dropdowns, I propose we use Font Awesome's fa-chevron-down in all of them, and that the hover and active states simply darken the border. Changing the style of our dropdowns can serve as a testing ground to see if we can remove the darkening of the fill color and the box shadow from our buttons.

It seems we have three types of dropdowns:

Dropdowns with a (pre)selected value
Dropdowns with a placeholder value
Dropdowns with an icon

They each need to use different colors for the content, but their borders and backgrounds should be the same.

The border color for hover state is #c4c4c4, which we already use in GitLab, and #c1c1c1 for the active state, which is a slightly darker shade that we don't use yet. They can be achieved as darken(#e5e5e5, 13%) and darken(#e5e5e5, 14%), respectively.

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Looks great to me @cperessini! Thanks!

I'd like to try and work on this issue.

Awesome. Thanks @deubeuliou! Let us know if you have any questions.

Milestone changed to %Backlog in gitlab-foss

@cperessini On the project home page, there can be a "Request Access" button next to dropdowns. Currently, they all look similar:

Did you mean to modify the dropdowns but not the "request access" button? Do we want all non-dropdown buttons to look like dropdowns? Should we first start changing only the dropdowns that are seen on the Issues page and see what to do next? (I'm going for the latter until further advice).

Mentioned in merge request gitlab-foss!7583 (merged)

@deubeuliou Good catch! We will eventually change those dropdowns too, but it'd look a little odd to have white dropdowns in between gray buttons. We're planning on removing all gray buttons from GitLab, but until that happens the approach you took is good, thanks!

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Mentioned in issue gitlab-foss#24145 (closed)

Good work @cperessini. I think dropdowns should look like normal buttons with the added chevron (hence the name dropdown buttons). We also have to look at how input selects look today to see if they need some kind of restyling.