Enable modsecurity in log-only mode (sysadmin#17870)

After migration to containerized Weblate, modsecurity started behaving
like crazy and the website was rendered unusable. After a couple of
days, we generated a list of matching rules and removed them in:

    9fcdd201

Among members of the Weblate team, we decided to completely disable
modsecurity in order to get the website back to a usable state. That was
done in:

    63766187
    cb7ff9fa

Now, among members of the Sysadmin team, we decided to turn modsecurity
back on but leave it in "log-only" mode, so we canin the future gather a
list of rules that match and decide how to go about them.