Extensions: Onboarding experience: Improve the authentication flow for instances that can't create access tokens

Created by: philipp-spiess

We soon have customers (those on free OSS on-prem installations) that will a custom URL but won't be able to create tokens.

With this change, we'll have three primary groups to consider for our onboarding experience:

1. Paying enterprise customers
   • Those will have a custom URL and be able to create access tokens.
   • In some rarer cases, they might not need an access token because they have a company-wide VPN with an http authentication proxy enabled
2. OSS users on sourcegraph.com
   • Those will connect to https://sourcegraph.com and don't need a custom token (but can still have them).
   • They have access to search all the open source code.
   • This user group is a good fallback for us since everyone can use this features regardless of their private code state.
3. OSS users on free on-prem installations

• These will have a custom URL to connect to but won't have the ability to create custom tokens.
• After the changes discussed in this document, these users will not be able to use the IDE extension and we should try to get them onboarded as 2) instead.

cc @muratsu @jjinnii @ryankscott @vdavid

Expected outcome of this issue

• Changes to the messaging on the notification that asks users to configure their instance URL.
• Potential changes to the settings page to make it clearer what authentication options are available.
• Discussion on what this means for our default URL?