Add block/activity inconsistent

The "Add a block" link is not consistent with the "Add an activity or resource" link:

1. Block has an underline on hover, Activity does not
2. The + icon used are the inverse of each other
3. Padding is vastly different
4. Add a block is not positioned in the middle

(EDIT to add rafaellechugo's proposal to description)

Changes to implement on "Add an activity or resourde" link:

• the roundness of the box around the "plus" icon should be defined by its correspondent bootstrap theme variables
• the box around the "plus" icon should have the same colour as the icon and the link (it looks clearer now)
• the gap between the box containing the plus icon and the text should be the same as the container padding (1rem or 16px)
• when hovering the container background gets slightly darker
• when hovering the container the text gets underlined

Changes to implement on "Add block" link:

• Move the plus icon to the left (however I don't think it should be printed inside a box, like its bigger counterpart)

• they both use the same icon now
• they both use the same padding now
• when hovering the container background gets slightly darker
• when hovering the container the text gets underlined
• It should not be printed inside a white "block"
• It should be printed against the page background and have the same width as the blocks