Access available features via Web Interface

[debuglevel]

• [x] The available features in the back end are accessible via the Web Interface.
• From now on, the Web Interface and the back end should have feature parity with no "hidden" features only available in the API.

[debuglevel]

• [x] Calculations: create
• [x] Calculations: read
• [x] Plans: partial read (deeper structures are missing)
• [x] Reports: download mail archive
• [x] Reports: view plaintext
• [x] Reports: create gmail drafts

[debuglevel]

Should have feature parity now for the main features in development.