Current file names have the form issue-42.yml which directly relates to the item ID.

Disadvantages:

• Does not give a clue about the file content beside the item type
• Files are not sorted correctly because there are no leading zeroes

Proposal: file names of the form 00042-slug-form-of-the-title.yml. The ID for references would still be issue-42 and can be derived from the file name and directory. This would also allow for shorter refs like "imp" (improvement) and "req" (requirement).

Disadvantages:

• ID is not directly visible from the file name
• IDs are not automatically unique, we have to verify this
• If the title changes, the slug may not be suitable anymore. I think we should not provide a UI for editing slugs for now. They can always be edited manually if desired. This would also not break refs because we use IDs for this.

(Related to #11)