Add a project description during blank project creation

Problem

Projects can have descriptions, but the description is currently managed outside of the settings area. There are two problems with that:

• Users are not asked to add a description during the creation of a new project. That means they might not even be aware that it is possible to add a description unless they stumble across it in settings.
• When a user wants to change the description, they have to know that they have to go into settings first, find the appropriate section and then change the description there. This problem is tackled in this issue: Allow editing of project description from the p... (#370470)

Current blank project creation page:

Proposal

Following what's been done in the new organization screen, introduce the option to add a project description to the project creation screen:

• This field should be optional.
• If a project description is added during project creation, it should immediately appear in the project overview screen upon creation of the project.

Implementation plan

There may be some shared functionality between this issue and #410817 (closed). See !146725 (closed) for a rough start to the functionality.

1. Remove include_description check in app/views/projects/_new_project_fields.html.haml#L50 and add mount element for Vue app
2. Create project_description.vue component in app/assets/javascripts/projects/new/components
3. Mount that component by creating a mount function in app/assets/javascripts/projects/new/index.js
4. Use app/assets/javascripts/organizations/shared/components/new_edit_form.vue#L212 as an example for the markdown field
   • preview_markdown route should already be setup for projects - config/routes/project.rb#L663
   • Move $options.restrictedToolBarItems to app/assets/javascripts/vue_shared/components/markdown/constants.js so they can be reused
   • Consider a reusable way to use the character count in multiple places. See discussion in gitlab-ui#1243 (comment 1800490947)