Bug 900539 - [SMS][MMS] Context Menu options for email address in message body. r=gnarf

- Changes "phone-link" to "dial-link" to match ActivityPicker.dial (allows directly mapping action to ActivityPicker function)
- Changes "data-phonenumber" to "data-dial" to match ActivityPicker.dial
- Consolidates duplicated MozActivity calls for email, phone and url links
    - Simplifies LinkActionHandler APIs to delegate to ActivityPicker wherever possible
- Removes duplicated try/catch boilerplate by providing an private abstract function for safely invoking MozActivity.
- Adds new locale string for "Send email" in email context menu
- Updates ThreadUI.activateContact to provide a "Send email" option, instead of a "Call" option when handling activations originating from an email address.

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>