Correct sorting by issues column when some entries are missing

[zbynek]

Adds data attribute used by sortable.js. Since data is always a number this guarantees sorting by number of issues will work as expected. Without this change, if one or more entries are missing in the column, alphabetical sort is used
2 > 12 > 1 > 0 > -

Also removes 2 lines of inline JS that is no longer needed, improving CSP compatibility.
Code introduced in 0a5457e8, made redundant in #1379.

Testing done

Manually checked that the column sorting works on my Jenkins instance and tooltip functionality is still OK.

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[uhafner]

Thanks!