Include a button icon on the login page for Bitbucket Server provider

[gtsiolis]

Problem to solve

Bitbucket Server (#8499) support was added in #8086, however, the login button for this type of provider is not using the button layout as the rest of the providers.

See #8086 (comment) and relevant discussion (internal). Cc @metcalfc @corneliusludmann @jldec

Proposal

This could be resolved by simply re-using the Bitbucket icon as the button icon for this type of provider.

[jldec]

@gtsiolis I think we need a button layout which is not truncated.
E.g. something more like the wider error-page auth button below (but consistent with the other buttons)

[AlexTugarev]

@gtsiolis are you working on this? it's a recurring thing mentioned on other PRs.

[AlexTugarev]

@gtsiolis, reusing the icon from Bitbucket seems fairly easy, right? The more interesting part is render the labels properly. Do you have a suggestion for that?

[gtsiolis]

@AlexTugarev I'm not actively working on any designs for this. Like you mentioned in #8603 (comment), reusing the Bitbucket icon is a great MVC (minimum viable change) for this issue. 🛹

thought: The truncation issue mentioned in #8603 (comment) is a different issue and can probably be tackled separately. Also, feature requests like #7061 could make the truncation issue obsolete.

suggestion: For resolving this issue, I'd suggest to simply re-use the Bitbucket icon and optionally use a title attribute for making the button more accessible. We could open a follow up issue to tackle the truncation issue separately. ➿