Update {% with %} syntax

[rafalp]

Django's {% with %} template tag used to have this syntax:

{% with something as other %}

Few versions ago Django introduced new syntax:

{% with other=something %}

We should update Misago's templates to use it.

Also, in few places where we nest with's, we should flatten them:

{% with lorem.ipsum.something as var1 %}
  {% with lorem.ipsum.dolor as var2 %}

To:

{% with var1=lorem.ipsum.something var2=lorem.ipsum.dolor %}

[paulinek13]

Hi! I’d like to work on this issue 😃
Is it still up for grabs?

[rafalp]

It is. I can assign it to you if you want. Also I'll be making one more issue to rename trans and blocktrans to translate and blocktranslate

[rafalp]

Rename for i18n tags is now tracked with #1853

[paulinek13]

Yes, please assign both issues to me. I'll work on them together and submit a PR within a day

[rafalp]

Fixed in #1854