Switch to gitlab domains config source completely (100% rollout) and disable the disk source

Description

We are slowly migrating toward GitLab domains config source. We are phasing disk source (config.json files) out.

This issue describes a point in the future were we completely switch to a new source, and no longer load config files from disk.

Rollout plan

• Pages v1.15.0 deployed to staging
• Check disk config source works OK on staging (#282 (comment 272245014)) (checked with 1.13)
• Check gitlab config source works OK for selected list of domains on staging
• Pages v1.15.0 deployed to production
• Check disk config source works OK on production
• Check gitlab config source works OK for selected list of domains on production
• Implement percentage based rollout strategy for gitlab config source
• New version (v1.15.0) with the percentage based rollout strategy deployed to production
• Increase rollout for gitlab config source to 5%
• Increase rollout for gitlab config source to 25%
• Increase rollout for gitlab config source to 40%
• Increase rollout for gitlab config source to 75%
• Increase rollout for gitlab config source to 100%
• [-] Remove code for disk config source
• [-] New version without the disk config source deployed to production

Follow-up issue #379 (closed)

Things to keep an eye on:

• gitlab config source cache hit/miss ratio (#TODO link to visualization)
• Pages internal API performance (visualization)
• gitlab config source HTTP client performance? (not yet implemented)