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

[gtsiolis]

/assign @trumbitta

[roboquat]

@gtsiolis: GitHub didn't allow me to assign the following users: trumbitta.

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In response to this:

/assign @trumbitta

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[trumbitta]

I mean, @gtsiolis, let's fix that 🤷😉😉😉😇