Refactor according to SOLID principles

[jonaskrask]

Currently, it is quite hard to test the extension, especially with automated testing. Therefore, we should consider refactoring the codebase to adhere to the SOLID principles. A tool that could be useful for this is inversify.

[c4rlo14]

The whole repository is currently around 725k LOC. Refactoring the codebase is an incredible amount of work and we don't think supporting automated testing is worth the effort in this case. In fact, we mainly test front end features manually.