Send Feedback using Github issues

[wadoon]

Intended Change

This PR adds a new menu item to create issues on GitHub directly from this repository.

This functionality uses the Github URL API; no API key is required, and the user's account is used. (It just prefills the issues form.)

Note: the length of URLs is limited.

It automatically inserts all Java files findable in the current JavaModel#modelDir, and the current KeYConstant.INTERNAL_VERSION.

Plan

• Implement it
• Test it by clicking on it
• Discussion

Type of pull request

• New feature (non-breaking change which adds functionality)

Ensuring quality

• I will made sure that introduced/changed code is well documented (javadoc and inline comments).
• I have tested the feature as follows: manually clicking the button.