Move Add to tree control

What does this MR do and why?

1. Create a separate component for Add to tree dropdown. The code is moved as-is from breadcrumbs.vue and breadcrumbs_spec.js into add_to_tree.vue and add_to_tree_spec.js
2. Change the position of the Add to tree dropdown, from breadcrumbs to next to Code button.
3. The old dropdown is kept still in breadcrumbs under directoryCodeDropdownUpdates feature flag. We want to bundle all changes to this area together, to avoid multiple controls shifting around.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Directory page - Update layout of buttons and rearrange actions into overflow menu
• Add WebIDE and Gitpod buttons to Compact Code Dropdown

MR acceptance checklist

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Screenshots or screen recordings

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feature flag off	feature flag on

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the feature flag

   Feature.enable(:directory_code_dropdown_updates)

2. Go to Project / Repository
3. Make sure the Add to tree dropdown is next to the Code button.
4. Check the layout on the narrow viewport.
5. In profile settings, change your default display of a project overview to Readme only.
6. Navigate to project overview, make sure the Add to tree is no present.

Related to #450666 (closed)