Add filter for `status/blocked-external`

[vmuzikar]

What this PR does:

• The bugs status now reflects the new status/blocked-external label.
  • Bugs with priority Blocker or Important now do not include bugs that have the status/blocked-external label.
  • There's a new column "Blocked External" that includes Blocker and Important bugs that have the status/blocked-external label.
• Added handling of logical OR for label filter.

[vmuzikar]

@stianst Thanks for the review. Done.