Improve session selection

[mgaonach]

Users with few associated proposals expect My sessions to show all their sessions, without having to select a date range.
This conflics with users having lots of proposals (for instance managers) for which showing all sessions would generate a too heavy request.

Idea: look at the number n of accesible proposals (/proposal/list) and:

• if n < x --> show all sessions and have optional date selection disabled by default
• if n >= x --> enforce date selection (current behavior)

--> test how much x should be to have a reasonable loading time.

This problem can later on be solved by server-side pagination when we break compatibility with java ispyb for EM & MX.