Add a Z badge to Zotero Connector icons

[adomasven]

See discussion in #501

On non-high-DPI screens we only have 16x16px for the icon, so I don't know how feasible at all this is, but maybe @yexingsha can come up with something.

[yexingsha]

Here are the badged icons, at both 1x and 2x, in light and dark mode: connector-icons.zip

Dark mode is necessary for the new icons since many of them (e.g. thesis) do not display well on a dark background.

To fit in the badge while maintaining legibility, I repositioned a lot of the original icons. manuscript, preprint, and standard got their distinguishing elements mirrored. Please let me know if there are icons missing, or if any of the icons look off.

[yexingsha]

Oh and here are the symbolic Zs: connector-z.zip

[adomasven]

Also need a collection icon as that is used for translators that can save multiple items.

[adomasven]

Also we need a webpage-gray icon for when there is no rich metadata.

For Chrome, Firefox and Edge: https://github.com/zotero/zotero-connectors/pull/502/files#diff-02d0b0120dc8a49b45570a18ebfc4355cbd23b211fafb5bdf55bae886a6bd588

For Safari I had to make a custom one which isn't quite gray, otherwise if Safari thinks an extension icon is too grey, it gives it a horrible blue tint: https://github.com/zotero/zotero-connectors/pull/502/files#diff-f4345d988c613ff6bb3668e8fdad8730170bd845ca79a56b7ab9274a560b2fa9

But maybe with the Z badge Safari will behave.

[yexingsha]

Here are the collection icons in light and dark; webpage-gray is already in the first zip file.