Bug 1052951 - [email] signature upgrade experience

Fixes three things:

* When no signature text, show "Tap to Edit" in gray for the button.
* When navigating to signature edit card, place the cursor at the end of the text automatically when the card is shown, so that it brings up the keyboard to show that it is editable. In order for this to look good, trailing whitespace is trimmed. Otherwise, br tags could get in, and the cursor would show up on the far right.
* For signature edit and for the compose field, allow break-word wrapping so that all text is editable on the screen. This does not insert real line breaks in the final text, it is just for display purposes.
* Fixes some small nits I should have caught before, about an incorrect card class name and that .tng-signature-input is also used for a different form control, so I switched the text edit field to be tng-signature-text.