Refactor import logos

What does this MR do and why?

This MR is making the import pages consistent, making the logos more visible, and aligning them with headers. It:

• increases logos size to make them more recognizable;
• uses sprite_icon instead of custom_logo for Gitea import
• adds bigger space between a logo and a title
• adds missing logo for the initial GitHub import page

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Checkout the branch
2. Visit the projects import page (*/projects/new#import_project)
3. Check the following importers
   • GitLab export
   • GitHub
   • Bitbucket Cloud
   • Bitbucket Server
   • FogBugz
   • Gitea
4. Validate that all logos are presented, under the #new and the #status pages 🎉

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.