fixing filters being reordered after editing

[LeXofLeviafan]

fixes #624 (as a workaround):

• now when clicking "Apply" in the list filters form, the filter numbers are automatically reassigned to match their order

also:

• when filter index is > 9, its index is automatically converted into a letter (since filters sorting is lexicographical); this is done during the submission, in the pagination links, and in the generated filters (filter_key())
• updated completions (for #777 changes)

Note: this also deals with the upstream bug mentioned in #777 (externally); though these two pull-requests might have a minor merge conflict with each other due to how git calculates changes (i.e. their modifications affect adjacent lines)

Example

 (changed a filter in the middle – this would normally cause it to be numbered last)  (after submission)  URL query (formatted for readability): ``` ?flt0_title_equals=a &flt1_title_equals=b &flt2_title_equals=c &flt3_title_not_equal=D &flt4_title_equals=e &flt5_title_not_equal=F &flt6_title_equals=g &flt7_title_equals=h &flt8_title_equals=i &flt9_title_not_equal=J &fltA_title_equals=k &fltB_title_not_equal=L &fltC_title_equals=m &fltD_title_equals=n ```

[jarun]

There's a merge conflict. Please resolve and merge.

[LeXofLeviafan]

@jarun fixed the conflict & updated completion files

[jarun]

Thank you!