Allow adding own deadlines

[vauhochzett]

The list is meant to be relevant for the chair, but that means that personally interesting conferences will not be listed. We could allow adding own deadlines, e.g. as follows:

• Have an upload feature for a locally created YAML file.
• Open a modal and let the user add all relevant info.

The data should be stored in a cookie and the conference added to the list under a category "custom" / "own" / ...