[#269] Switch to activeTab permission

[maltoe]

Tested on Chrome 88 and FF 82. Ticket is recognized both when clicking the extension icon and when pressing Ctrl+T.

Unfortunately wasn't able to find reliable information as to when support for activeTab was added to the respective browsers, but at least all current versions support it.

• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions#activetab_permission
• https://developer.chrome.com/docs/extensions/mv2/manifest/activeTab/
• https://dev.opera.com/extensions/declare-permissions/
• https://developer.apple.com/documentation/safariservices/safari_web_extensions/managing_safari_web_extension_permissions

Further restricting the permissions to hosts (e.g. github.com) would make sense for cloud services, but could be difficult for self-hosted providers like GitLab, so I did not pursue this further.

Closes #269.