Update footer INN logos

[tylermachado]

This adds in two new files for the updated INN logo, in the new blue (when on light backgrounds anyway).

Closes #1631

[benlk]

Is it possible to optimize those images even more than their current filesize allows?

[tylermachado]

I thiiiink I tried to get them as small as possible? Considering for pixel size and transparency, at least

[benlk]

I converted these from color PNG to indexed PNG, with max 127-color palettes, and got the image sizes way down. Can you take a look and see if these images are acceptable?

35K inn_dark.png inn_dark (original)

7.5K inn_dark-ex.png inn_dark-ex — just as harshly aliased as the inn_light-ex logo below, but perhaps less apparent on the backgrounds i've been previewing on.

32K inn_light.png inn_light (original)

9.8K inn_light-ex.png inn_light-ex — harshly aliased, but we're usually showing these images at much lower display sizes than their intrinsic size.

[tylermachado]

I inserted those into my local install and I felt like the aliasing was noticeable even at a width of about 300px. Maybe because I was just looking for it going in? That said ... what if we reworked the code a touch so that it took an SVG instead?

[benlk]

https://caniuse.com/#feat=svg

Switching to SVG would be awesome.

[tylermachado]

Ok, made SVG files for this round. I embedded the SVGs in tags -- the PHP file_get_contents() method was throwing SSL errors that I couldn't decode, and I'm not sure how that'd play with the suggestion to use functions.php to swap the colors out.

[benlk]

Merged latest 0.5-dev branch into this, updated a line of docs, updated the changelog.

Notes on page size, with INN_MEMBER set to true:

• before this PR, 251 KB
• in this PR, 248 KB

Questions which we should think about before merging this PR:

• Would we still want to include the .png files in Largo? The ones introduced by earlier commits in this branch are kept by the commits that added the .svg files; should we keep the .png files around as reference images or just remove them?
• Should any of the documentation here link to INN's brand standards? If so, should these docs include a request that, if someone notices a discrepancy between the brand standards and Largo, that that person file an issue in this repo or send an email?