Use PageSelector for manage repositories page and other fixes

Created by: artemruts

Changes

Closes #18780 (closed), closes #19367 (closed)

• Swaps custom pagination for shared PageSelector component
• Selected repositories are displayed at top of list in subsequent loads
• Fixes broken pagination when using query search
• Displays code host errors
• Normalize case when searching for repos

⚠️ We'll have to wait for https://github.com/sourcegraph/sourcegraph/pull/19378 to land to start using PageSelector

TODO

• fix slight layout shift due to scrollbar visibility on async data load (#19371)