battle_calcs.lua: fix bug in get_attack_map_unit() for enemy units

Own units with MP left need to be taken off the map before enemy path
finding, not just added to the table.
add set_action_bonus_count when attacking

Duthlet noticed this was missing.
fixup fe722172ae (add set_action_bonus_count)
Fix bug #21967: fix crash when unit modification to traits has empty id
wmllint: change "usage of" magic comment to re.search

There seems no particular reason to require that this magic comment be at
the very beginning of the line, so why not switch from re.match to
re.search.

Also, update comments to reflect the fact that UtBS no longer uses this
magic comment.

A suggestion has been made to get rid of this magic comment now that UtBS
is no longer using it, but it may still be in use in some UMC somewhere.
html.py: format add-on description for HTML

The description text does not get rendered very well on a webpage. One
solution might be to use pre-wrap/word-wrap in the CSS, but due to
differences between browsers, that's a can of worms (at least for me, I'm
not a web pro).

So, the not-so-elegant solution is to add <br/> to every line.

URLs are also not linked in the plain text. Although in modern browsers
you can select the text and right-click, it's still convenient to turn
them into actual links.
html.py: import re module

Bugfix for last commit.
add remark about 21965 to RELEASE_NOTES
remove spurious WML variable manipulations in playcampaign.cpp

these appear to cause replay corruption, and bugs 19218, 21562

the replay_start cfg is already set correctly as the snapshot
at the start of a game, in playsingle_controller.cpp, in the
play_scenario function:

https://github.com/wesnoth/wesnoth/blob/0aa13d477d156929616a7409010474c4dc6b5439/src/playsingle_controller.cpp#L372
update changelog