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 implementing search-with-markers #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

[LeXofLeviafan]

More specifically, there's a chance of this (or the next) change causing a merge conflict since the other pull-request modifies extra_css.

[jarun]

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

[LeXofLeviafan]

@jarun fixed the conflict & updated completion files

[jarun]

Thank you!