Replace PDFs with SVGs from desktop/.../assets where available

[acb-mv]

This replaces PDF images with SVGs where the desktop assets had suitable ones. A few things are unreplaced (mostly button shapes). The elements may appear smaller on screen due to the SVGs not being cropped as the PNGs were (the PNGs seem to have been handcrafted for the app and not mechanically derived from the SVGs).

This is what the main screen looks like:

and the account screen:

---

This change is 

[linear]

IOS-1101 Migrate existing icons from PDF to SVG format

[rablador]

Some icons need adjustment. I think we should fix that in this PR.

Reviewable status: 0 of 72 files reviewed, all discussions resolved

[pinkisemils]

Specifically, the arrow for the login text field should be adjusted so that the whole text field is not blown out of proportion, and the cog and account icons maybe should be increas in size? @waahlnaden probably has some ideas.

[rablador]

Reviewed all commit messages.
Reviewable status: 0 of 72 files reviewed, all discussions resolved

[acb-mv]

I have adjusted HeaderBarView to fudge the sizes of the button images to compensate for SVGs not being trimmed.

Reviewable status: 0 of 73 files reviewed, all discussions resolved

[acb-mv]

I'm looking at the login screen now, and it looks like the close image (which has a larger intrinsic size) is blowing it out, not the arrow. The desktop assets had only one close image size, whereas we had the same image with different dimensions.

I think a solution may be to replace our uses of UIImage(named:"...") with an extension on UIImage that produces a UIImage.closeSmall whose dimensions are suitable for our purposes, but doesn't burden the caller with caring about where it gets the image from.

[rablador]

The header icons look good, but we should fix a couple more:

Reviewable status: 0 of 73 files reviewed, all discussions resolved

[acb-mv]

A lot of those are part of the close icon regression. I'm in the process of refactoring how we retrieve asset UIImages, which will fix this.

[acb-mv]

Our image assets are now loaded and sized as appropriate in UIImage+Assets.swift, which fixes most of these.
The checkmark in the filter edit dialog is still a bit more slender than the PDF one, though.

[rablador]

Looks a lot better now, checkmarks included. However, the close buttons are too small now, and the info buttons too large. What do @waahlnaden think?

Reviewed 70 of 72 files at r1, 27 of 27 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved