Refs dropdown is now loaded async

What does this MR do?

The refs dropdown is loaded async so not to block the page.

What are the relevant issue numbers?

Part of #18202 (closed)

Screenshots (if relevant)

Added 162 commits:

• e7d6fa71...915ad255 - 160 commits from branch master
• cb363fc0 - Refs dropdown is now loaded async
• 60da76f9 - Tests fix for ref switcher

Added 1 commit:

• c2108103 - Tests fix for ref switcher

Added 1 commit:

• 09519261 - Escapes branch names before appending to dom

Added 1 commit:

• 23770de8 - Pulls back tags if any exist

Added 109 commits:

• 23770de8...8578f401 - 105 commits from branch master
• 6cd049e8 - Refs dropdown is now loaded async
• 73225745 - Tests fix for ref switcher
• 29cfe058 - Escapes branch names before appending to dom
• ffc618b5 - Pulls back tags if any exist

Added 32 commits:

• ffc618b5...30ee4ea6 - 28 commits from branch master
• 9604ad2f - Refs dropdown is now loaded async
• c8f231fa - Tests fix for ref switcher
• 0026a49e - Escapes branch names before appending to dom
• 8a3b13b5 - Pulls back tags if any exist

@iamphill let's get these tests fixed, I really want this to get into 8.9, I think it's going to make a big difference.

Actually, the failing tests were due to the bug in master. So this should pass once we rebase :D

Added 78 commits:

• d00e20d1...cea3cf17 - 74 commits from branch master
• 8de99c9e - Refs dropdown is now loaded async
• 9e108fe1 - Tests fix for ref switcher
• 4d9bd72b - Escapes branch names before appending to dom
• e8707023 - Pulls back tags if any exist

Added 6 commits:

• e8707023...e0f3e44b - 2 commits from branch master
• 72a83b61 - Refs dropdown is now loaded async
• ecf45bcd - Tests fix for ref switcher
• fbddd717 - Escapes branch names before appending to dom
• 53bd9fa7 - Pulls back tags if any exist

Unmarked this merge request as a Work In Progress

Milestone changed to %8.9

Reassigned to @jschatz1

@jschatz1 I think this is going to be a pretty significant performance boost for some pages, can we prioritize getting this in 8.9?

@rymai can you review Ruby? I still need to review frontend stuff.

Reassigned to @rspeicher

Reassigned to @rymai

Added ~149423 label