Add global configuration UI.

[sopel]

ℹ️ - This only summarizes what has been communicated via respective mails already, but capturing it here as well will help to manage release 1.0.0, which might or might not include this improvement depending on stakeholder feedback/priorities.

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In contrast to all other White Source CI agents, the Bamboo agent currently doesn't feature a global configuration UI (a shortcut from the early project phase). While it is possible to achieve the same effect for the API key by using a Bamboo variable, a hallway usability test suggests this to be less accessible for infrequent Bamboo users at least - given all other agents/plugins expose a dedicated global configuration it seems appropriate to add one for Bamboo as well.