Remove modals and buttons that summon them from MR widget

UX @dimitrieh FE @fatihacet

Issue idea came from: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9253#note_24107989 by @jschatz1

It will need #25424 (closed) to be finished first or at the same time

Problem

With #25424 (closed) we do not yet attend these buttons:

which can summon this modal:

Modals are not the best UX, let's see if we can change this for the better.

Proposal

Replace the modal with a better solution

Design

• remove "check out branch" button
• add copy to clipboard button for branch
• converting download button to an icon button
• Clicking Expand or Collapse after "command line instructions" will expand and collapse this section which was previously in "check out branch" button at the bottom of the merge request. (similar functionality as to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11379)

Value

no modals, quicker access to complete its usecases

Related issues

#25424 (closed)

CC/Call to actions

@filipa @grzesiek @godfat (do you use these buttons? if so, what for? For example I only copy the first box to my clipboard to get to the branch in my local git repo)