All Training Requests view: simplify creation of form vars, remove redirection
All Training Requests view: simplify creation of form vars, remove redirection

`form` and `match_form` variables are created on top of the view,
and get overwritten as required further below.

Redirections were removed because we don't need to remember filter
parameters across requests - they're going to be preserved:
1. by adding `?{{request.GET.urlencode}}` to <form action>
2. by saving checked checkboxes in browser's JS `sessionStorage`
3. by saving table sorting/paging/filtering parameters in browser's JS
   `localStorage` (Datatables feature).

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