css: Use SCSS nesting for `.alert-notification`.
css: Use SCSS nesting for `.alert-notification`.

We also remove the `#settings_page` container as there
are plans to reuse the above class in the stream edit
page as well.

eeshangarg/zulip,rht/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,punchagan/zulip,rht/zulip,shubhamdhama/zulip,andersk/zulip,hackerkid/zulip,shubhamdhama/zulip,brainwane/zulip,rht/zulip,eeshangarg/zulip,showell/zulip,synicalsyntax/zulip,showell/zulip,brainwane/zulip,shubhamdhama/zulip,shubhamdhama/zulip,showell/zulip,timabbott/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,punchagan/zulip,shubhamdhama/zulip,timabbott/zulip,punchagan/zulip,shubhamdhama/zulip,hackerkid/zulip,timabbott/zulip,synicalsyntax/zulip,eeshangarg/zulip,showell/zulip,showell/zulip,rht/zulip,rht/zulip,synicalsyntax/zulip,kou/zulip,andersk/zulip,brainwane/zulip,zulip/zulip,synicalsyntax/zulip,brainwane/zulip,brainwane/zulip,hackerkid/zulip,timabbott/zulip,punchagan/zulip,rht/zulip,showell/zulip,brainwane/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,brainwane/zulip,zulip/zulip,punchagan/zulip,showell/zulip,timabbott/zulip,kou/zulip,eeshangarg/zulip,synicalsyntax/zulip,rht/zulip,timabbott/zulip,synicalsyntax/zulip,andersk/zulip,zulip/zulip,synicalsyntax/zulip,kou/zulip,shubhamdhama/zulip,zulip/zulip,timabbott/zulip,eeshangarg/zulip,punchagan/zulip