Search page improvements

[benlk]

Changes

• Search results pages gain the archive class to get a lot of the styles we use in term archives.
• Search results lose some custom per-article styles.
• Default Largo search form in the main column of the search results page now has markup matching the header search form.
• Search form in the main column of the search results page is now 100% width of column, with submit button at 10% width and the input at 90%. Also added: 0.5em padding-bottom, 1px dark grey border-bottom, 1em margin-bottom to put some space between the header and the results.
• Excerpts in the content-search partial now default to 2 sentences instead of 5, matching the limits used elsewhere in Largo. (Not shown in the screenshots below.)

## Why

For #1162 and RNS-198.

[benlk]

Julia, what feedback do you have?

[aschweigert]

the mobile view is a little odd with the search icon in the header and then the search box...we probably don't need both. I'd also consider slimming the results down even a bit more, probably no top tag, maybe no byline, a very short excerpt if possible. Smaller headline (probably), generally just make this a bit more compact and more search results-y

[benlk]

I dunno, I think the longer excerpt is better for search results, in helping people find what they're looking for. Shorter excerpts feel clickbaity.

• is removing the search icon in the header possible without a hideous conditional/css?
• remove top tag
• comment out byline (leaving placement there in case child themes want the byline)
• shorter excerpt, to 1 sentence or the custom excerpt
• remove the thumbnail

[benlk]