Add GET endpoint to extension API

[hursey013]

Allows checking if a bookmark already exists from the extension by using:

https://shiori.domain/api/bookmarks/ext?value={{encoded URL}}

This paves the way to allow the extension to see if a bookmark exists without relying on a local browser bookmark folder.

[fmartingr]

Hey @hursey013 , thanks for the PR!

A few notes before moving forward:

• Rebase the changes from latest master, since the session duration was already implemented by where is the shiori docker database? How to backup and restore #446 the changes in the assets and login files are no longer required.
• We can use status codes instead of payloads. So, instead of returning a bool we can just use the 200 and 204 status codes.

Please let me know if you're (or not) interested in moving this forward so I can prioritise it myself.

[stale]

This issue has been automatically marked as stale because it has not had any activity for quite some time.
It will be closed if no further activity occurs.
Thank you for your contributions.