Exceptions: Rework and basic Sentry integration

This adds basic Sentry integration. Sentry allows Gajim developers to gather tracebacks from Gajim users without them having to register on Gitlab.

Some advantages:

• increased number of reports
• automatic issue deduplication
• counting: prioritize issues
• possibly more (structured) context

What has been done in this MR:

• "Send Report" will initialize sentry_sdk and afterwards send the traceback to the specified endpoint
• Users can view the report before sending anything
• Users have full control: "Close" will dismiss the window, nothing will be sent
• Endpoint updating (client part)
• Endpoint updating (server part)
• Add more context (depending on endpoint capabilities)
• Add user input (depending on endpoint capabilities)
• Add sentry_sdk as an optional dependency (Readme)
• Add sentry-sdk dependency for Windows builds
• Decide on Sentry endpoint: self-hosted instance / Gitlab integration (very limited at the moment)