Add support for a bookmarks-only mode

[agologan]

This adds the ability to disable manually-entered connection details in favor of bookmarks.
Feel free to recommend any changes that better align with project directions.

The use-case for this self-hosting and avoiding users to ever provide connection details.
A connect backend is sortof similar, but bookmarks already go 90% of the way and come baked-in.

[sosedoff]

Thank you. I will have a look

[sosedoff]

When pgweb bookmarks dont exist i get this UI:

After clicking on the "Connect" button:

In this particular case i think its best if we can display a message like "No bookmarks available" and an explanation
that Pgweb is running in bookmarks-only mode and connecting to custom databases is not allowed.

[agologan]

Hid the connect button and showed an error reading:

Running in bookmarks-only mode but NO bookmarks configured

Also added errors when attempting to start with URL or host + port or connect-backend set.

[sosedoff]

Looking good so far. Left a few comments, i will test a bit more and merge if everything is in order after you make the changes.