Provide a new area for actions in blob view to minimize gutter
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### Proposal
The gutter on the blob view is very large. The gutter provides a spot for actions related to the selected line.
**Propose change:** Consolidate actions for lines of code in a dropdown menu accessed via a more actions button (icon button with `ellipsis_v` icon).
- Actions are more clear by using the words "View blame" instead of a git icon.
- The gutter can be narrow to fit the line numbers and avoids having a large gap currently
- For "Explain this code", if the user clicks on the action but there is nothing selected then it will assume that the selected line is the line of code to explain.
- Provides an area where we could add more actions to in the future should we pursue additional actions
- On hover, the menu for actions should appear
Button should appears when the user hovers in the gutter area and/or the line of code. For reference, an example of showing actions on hover of the line of code exists in merge requests where the suggestion action appears when the user hovers over the line.
| Current | Proposal |
|---------|----------|
|  |  |
[Open in Figma](https://www.figma.com/file/eq2nf3B8MhB9xKrC0VzbPf/branch/dcjLR3ECXG5chUQShAmVis/Repository?type=design&node-id=637%3A154030&mode=design&t=zq0C9FSrAFPfTT0R-1)
### States
| Hover over code | Hover in gutter | Menu open |
|-----------------|-----------------|-----------|
|  |  |  |
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### Additional design details needed
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### Implementation notes
Instrument this feature to gather data on usage per product request in https://gitlab.com/gitlab-org/gitlab/-/issues/15431#note_2354456783.
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