fix: Crawler proxy selection fixes

[SuaYoo]

Fixes #2168

Changes

• Hides proxy form control if there are no proxy servers available
• Fixes org default proxy value not being saved

Manual testing

1. Log in as superadmin
2. Select org with proxies
3. Go to Settings -> Crawling Defaults. Verify "Crawler Proxy Server" is available
4. Update "Crawler Proxy Server" and save
5. Go to Crawling -> New Workflow -> Crawling Settings. Verify correct default proxy is shown
6. Go to Browser Profiles -> New Browser Profile. Verify "Crawler Proxy Server" is available
7. Select different org without proxies
8. Repeat 3-6, only verify that "Crawler Proxy Server" is not visible and forms save as expected

Follow-ups

Browser profile proxies need some updates, see #2281

[emma-sg]

Nice to see us start using @lit/context! I take it #2281 will be addressed separately?

[Shrinks99]

Tested, works great!