Validate collection page URL

[SuaYoo]

Resolves #2286

Changes

• Disables saving collection start page if valid snapshot is not selected
• Shows full URL in page URL status check mark
• Shows error in page URL status exclamation mark
• Fixes pasting in URL

Manual testing

1. Log in as crawler
2. Go to collection with items
3. Click "Configure Home"
4. Choose "Start Page"
5. Paste in full URL of a page that's in the collection. Verify snapshot is selected by default
6. Hover over success checkmark. Verify full URL is shown
7. Clear page URL input. Verify snapshot is also cleared
8. Enter page URL that isn't in the collection
9. Click outside of page URL input. Verify input is highlighted in danger color and "Save" button is disabled
10. Hover over !. Verify error message is shown

Screenshots

Page	Image/video
Collection detail - Initial state
Collection detail - page exists
Collection detail - page not found

[SuaYoo]

Could maybe also be a small 404 badge? Feel free to update if you have ideas @Shrinks99

[Shrinks99]

I like the exclamation point, but would rather not add additional icons to the right side of the input field where the clear button is (which is an actual intractable control). Could we show the error text below like we do for other form fields and maybe swap out the search icon on success / error?