Revert GitHub WebBrowserUrlProvider extension

This extension point has semantics that is different from provided by these commits.

Revert "Github: add WebBrowserUrlProvider". Commit f27a8ea87be16763259a0c5d948891a850e03b0c.
Revert "Github: do not collect additional data in quiet mode". Commit 0a63cd4a97e4eef53be6aba6438a509636df8ed6.
Revert "Github: update action quietly". Commit d83c4e8189bd3eb013380541226184a5efea1795.
Remove code duplication
Github: add computeValueInModal without exceptions
Github: add util function
IDEA-112867 Github: show diff for remote branch

* add remote if not exists
* fetch upstream before diff
* ignore case only in user-input strings
Github: run in EDT
use not-null CachedValue dependencies