add the capacity for user to select arrows used for a stats session

[remace]

an archer, in a stats session or in a practice session, often has a lot of arrows (more than 10).

the navigation while entering scores or stats would be easier if a list of "concerned arrows" is displayed: the choice would be faster and easier.

solution:

• a many-to-many relationship between arrows and session
• each entry view renders it in a template, within a hidden div
• the javascript gets the list in the template and proposes it to User when needed

[remace]

done in c025793