make easier to customize logo sizes

[evanwill]

tweaking logo height/width is probably necessary in most collections, currently surprisingly fiddly to do.

• add id to home banner logo img
• add id to footer logo img
• add method to quickly tweak pixel dimensions (is this in _custom.scss or theme.yml or next to the logo image src in config.yml??)

[evanwill]

maybe even just instructions like "add #footer-logo { max-width: 140px; } to _custom.scss"?

[evanwill]

I think it might be worth changing _config.yml options to have:

organization-logo-home
organization-logo-banner
organization-logo-nav
organization-logo-footer

it is a lot, but would make make simpler to customize, since it is a bit ambiguous now (i.e. -banner currently is on non-home page banner, -nav is in footer, homepage, and mobile nav--> but these decisions don't necessarily work if you change the theme at all, and if your logo is differently shaped they aren't all interchangeable in that way)

[evanwill]

actually, maybe remove the "organization" part, just "logo-", since we don't need to assume it is an org (thinking of context of CB-GH, where currently we have a different banner and footer without org logo and links). It could just be a collection logo or not used at all, too.

[evanwill]

current bits of CSS notes:

// in _base.scss
#mobile-nav-image {
    max-height: 1.5em;
}

#footer-logo {
    height: 40px;
}

// in collection-banner.html
#home-banner-logo { 
    height: 40px; 
}

stuff in collection-banner.html added to be able to use Liquid to set values without complicated set up to pass values through the main scss.