Turn on all of Microsoft's recommend lint rules

Biggest changes forced by this: alt tags for all images, resulting in
new strings added to messages.json, and a new i18n paramter/prop added
in a plot of places.

Another change of note is that there are two new tslint.json files under
ts/test and ts/styleguide to relax our rules a bit there. This required
a change to our package.json script, as manually specifying the config
file there made it ignore our tslint.json files in subdirectories

ts/types/Attachment.ts

@@ -121,6 +121,7 @@ export const getSuggestedFilename = ({
     : '';
   const fileType = getFileExtension(attachment);
   const extension = fileType ? `.${fileType}` : '';
+
   return `${prefix}${suffix}${extension}`;
 };
 
@@ -133,7 +134,6 @@ export const getFileExtension = (attachment: Attachment): string | null => {
     case 'video/quicktime':
       return 'mov';
     default:
-      // TODO: Use better MIME --> file extension mapping:
       return attachment.contentType.split('/')[1];
   }
 };