Improve time entries export

Time entries are a nice addition to Freshrelease. The way one can export his/her time entries could be improved:

1. As of now, only a project admin can export them; all users should be able to do so.
   
2. When exporting the time entries, one should be able to select the fields he/she wants to appear in the CSV; right now, it's arbitrarily decided to only export a few fields.