Take no action for middle click over pinned tabs

[cezaraugusto]

Test plan

1. Pin 2 tabs, have 1-2 unpinned
2. Middle click over a pinned tab
3. Should do nothing

Description

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Auditors: @bsclifton
Fix #9492

repeating comments left in #9492 (comment):

Middle click over pinnedTabs shouldn't be allowed until #4063 is solved. Doing it was leading to a weird update over tabs, setting unpinned tabs to the smallest breakpoint, which is visually similar to pinned tabs but with close button replacing icon while active.

for clarification, this pull-request copy the same behavior as Opera, which may not be our intention given FF, Chrome and Safari all close the tab. However, leave the discussion open until #4063.

/cc @srirambv

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

Works great! Nice fix 😄

Good find, @srirambv!