Keep input values on validation error of expression in alert profiles

[johannaengland]

In #2667 validation of IP addresses of expressions in alert profiles was added. This could be further improved (probably by restructuring alert profiles further):

1. If I select "in match" from the dropdown and enter an invalid value, my dropdown selection is reset to "equals match" when the form displays the validation error.

2. Also, is my value supposed to be blanked out from the form if it was invalid? What if I input a really long string and only had a small typo? Now I have to enter everything all over again, rather than just fix the one typo...

Originally posted by @lunkwill42 in #2667 (review)