SharedLinkProvider cleanup

[henryptung]

• Remove SharedUrl paste-in class, keep necessary bits
• Remove ownCloud-specific handler (can mangle non-ownCloud links too)
• Push objects to storage one at a time
• Do not attach objects to this before storing (avoids vue.js contamination)

Fixes #307

[henryptung]

Important note: I didn't port over the ownCloud link handling because it seems prone to mangling legitimate non-ownCloud links (i.e. any alphanumeric folder with exactly 15 chars will get a /download stuck on the end). I can add it back if desired, but just FYI.

Also, for the file-path check, the old check looked like it only forbade URLs with no path at all (e.g. http://example.com and allowed URLs with any path at all, e.g. http://example.com/. That didn't seem very meaningful (why refuse the former if you're going to accept the latter anyway?), so I went out on a limb and assumed it wanted to block "directory-like" urls (i.e. ones ending in /) - if that's mistaken, I can implement whatever check is desired here (or remove the check altogether).

[mishley]

Just wanted to confirm that I'm seeing this same behaviour. I use a permalink via Nextcloud, and it's currently acting just as @henryptung described in his bug report.

[henryptung]

@subdavis Any thoughts or concerns with the changes?

[jfcardinal]

I'm going to merge now, test, and push a batch of changes to the store eventuallt.

When is that going to happen? I just found out that Shared Links don't work in Firefox after using them in Chrome. Firefox is my preferred browser, and I was using an old version--with legacy extensions--until now.