Highlight and pin the default branch for projects from Bitbucket

[gtsiolis]

Problem to solve

Following up from #7251 (comment):

... we 🅰️ don't have the default label for the default branch and 🅱️ we don't pin the default branch at the top of the branches list.

Branches for a GitHub repository	Branches for a Bitbucket repository

Proposal

Let's 🅰️ introduce a label for the default branch for Bitbucket repositories and 🅱️ pin the default branch at the top of the list as done with GitLab and GitHub repositories.

[jldec]

/schedule