Support Redmine 6

[dkastl]

Changes proposed in this pull request:

• Supports Redmine 6
• Uses SVG icon for admin menu
• Updates dependencies
• Clean up of some hooks

Not sure if these changes still work with v5. Needs to be checked.

[sanak]

@dkastl Thanks a lot!
I confirmed that all tests passed in this branch.

Supporting Redmine 5 seems to be possible with slight change, so I created another PR (#339) to this PR branch.

[dkastl]

I forgot that I also need to check for API updates/changes.

[dkastl]

@sanak , I updated the API templates and reviewed the README.
If you are fine with the changes so far, then I will merge into next.

[sanak]

@sanak , I updated the API templates and reviewed the README. If you are fine with the changes so far, then I will merge into next.

Thanks!
LGTM about the diffs (9cdc7b5).