Restructure MR merge widget
This is an actionable insight from the [UX Department: MRs experience async critique](https://gitlab.com/gitlab-org/ux-research/-/issues/1073) research ([insight][insight]).
### What
In the **merge** section of the MR widget, the “merge commit message” field is below the “Merge” button, so one has to navigate up to submit after filling in the field. The same thing happens to the “Delete source branch” and “Squash commit” options.

### Why
Form submit buttons usually come after the form fields, unless they are sticky/fixed to the screen. In this case, the user may be confused as to where they should click to proceed.
### How
Move the merge options before the “Merge” button, so that it follows the logical order of filling->submitting. A design for this was first explored in https://gitlab.com/gitlab-org/gitlab/-/issues/299195 and this issue aims to be the development iteration to get this in the product.
The main aspects of the proposal are:
1. Re-order merge options and action — `(R6 * I2 * C1)/E1 = 12`
1. Move allow collaboration alert to branches widget
1. New placement for merge/rebase errors
1. New SHA mismatch state
[:frame_photo: **See Figma file**](https://www.figma.com/file/33KrJ7p9eTHlneMiSBDf57/Restructure-MR-merge-widget-293653?node-id=1%3A2)

[insight]: https://dovetailapp.com/projects/14109fbc-dfdd-4204-a54e-9c80ba8b7bba/insights/e9489d24-fbc1-4138-bf9a-c4ed745ecb2a
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