Implement schematisation descriptions

[leendertvanwolfswinkel]

We want to start using descriptions on a Schematisation level. The description is stored in and read from the meta property of a schematisation ("meta": {"description": "my very cool description"},). Currently, the meta property of schematisations do not yet contain this key, so take this into account.

Please implement this in the following components:

• [ ] New schematisation wizard, first page, between 'New schematisation name' and 'Tags', add 'Description'. The grey hint-text in the text input widget should read 'Concise description of your schematisation (optional)'
• [ ] Download schematisation dialog, remove the 'slug' column and replace it by a 'description' column

[leendertvanwolfswinkel]

@ldebek I tested this. The New schematisation wizard changes work as expected. The download schematisation dialog has a "description" column, but does not display the schematisation description.

[ldebek]

@leendertvanwolfswinkel Fix for the issue pushed.

[leendertvanwolfswinkel]

Perfect, please merge