Updated github issue templates

Updated github issue templates

- Removed titles since they don't provide added benefit upon the labels
  and would often lead to being submitted with just the placeholder
  title.
- Feature request form
  - Added further context to benefits field for hopefully better
    responses that target the core goal.
  - Added a field to ask if feature can already be achieved, to
    gain an idea if the submitter has explored other options (if
    existing).
  - Added a field to ensure the submitter has search the issue list
    before submitting.
  - Added a field to ask existing BookStack usage time to understand
    potential evolution of usage and/or influence of other platforms.