Improve text in "new promotions" form for better usability

[Mandily]

Related to the improvements in #1702, there's some improvements to the language used on the promotions form that will make it more intuitive for users to use.

• Remove the "General" title from the top of the page. It doesn't add any value to the page, and takes up extra room.
• Add info text to the description "Limit of 255 characters" - Bonus points if we can add a character counter to the description. If we can do this, we can get rid of the text and just write [number of characters used}/[max number of characters] (ie 0/255)
• Change the label "Add rule of type" to "Discount rules"
• Change the label "Add action of type" to "Adjustment type"
• Change the label "Base Calculator" to "Discount type"
• Add the currency symbol into the grey box prepended to the currency input
• 

[MFRWDesign]

@MFRWDesign is working on this

[kennyadsl]

This can be closed after #2917.

• Bonus points if we can add a character counter to the description. If we can do this, we can get rid of the text and just write [number of characters used}/[max number of characters] (ie 0/255)

is still to be implemented but I think it won't be done for now.