[Proposal] Improve file names

[erikhofer]

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)

[erikhofer]

Experience in real-life project: item titles are changed regularly. Sometimes wording is improved slightly, sometimes the title is changed copletely in the course of iterative refinement. At the moment, it feels good to not have to worry about file names and evolve titles freely.