Solved the problem of <p> tags being given different padding or color by different clients, without having to copy-paste a string of CSS into every <p> tag in the templates.
The greeting includes the recipient's user name (as for the message notification mail), which should increase the recipient's trust that the message isn't spam. Greetings were reused from each language's respective locale flie.
The tests assumed that every part in the multipart emails that we send were either plain text or HTML. Now we have image attachments, against whose contents the tests were still trying to match regexes. The tests have been modified to only run regexes on the text parts of the mails.
Several notifications are about a message having been sent from one user to another via OSM. I've moved the code for the HTML table that holds the actual user message, with the avatar and the body text, to its own partial template, so that it can be reused. I've updated a second notifier message to the new template, diary_comment_notification.
Since all HTML mails will presumably share the same overall layout, and the markup is fairly dense, it makes sense to use Rails layouts to share the structure across mail templates. This commit moves the core structure of the HTML mail to a layout. It will need to be further refined so that notifications that involve a text message sent by another user can share the avatar-and-message-text structure.
Rather than including HTML markup in the locale file (which also has the inconvenience of requiring both a plain-text and an HTML duplicate of the same string), move the markup to the template. Also added a helper to reduce clutter in the template slightly.
Make the file paths to image assets more compact. I investigated using `image_path` but could only get it to return the path for a public URL, which is different and also includes the asset pipeline digest.
Got rid of that big ugly base64 blob which was only a temporary crutch anyway. Added a png file instead. Use File.read rather than File.open for a 66% reduction in verbosity.
Removed some cargo-cult HTML attributes and CSS. Styling HTML for email clients is trickier than for browsers, I'm trying to keep the code as succinct but also as compatible as possible.
