Create new group: Rename form fields and update UI

Problem to solve

Via https://gitlab.com/gitlab-org/gitlab-ce/issues/2934#note_98403973 we are renaming project-specific labels to Project URL and Project slug when creating a new project. The same is needed for groups for consistency.

Proposal

• Rename labels on "New group" page.

  • Group path to Group URL
  • Description to Group description

• Group slug gets generated from Group name.

• Also, let's position the fields similar to the "New project" page.

  • New forms layout (labels on top, bold)
  • Add "(optional) to the Group Description label
  • "Group name" to the top

Current "New Group"	New "New project"

Solution

Normal	All errors	'Group path taken' error

Additional suggested changes:

• can we autofocus the 'Group name' input field?
• remove the help icon from the visibility section and replace it with a description + text link.
• we need to indicate that the number of characters allowed for group description is limited. I created a separate issue for the design pattern. Let's add this as a separate issue afterwards.