Hemi logos and footer links

[gabmontes]

Description and Related Issue(s)

Fixes #8.

Proposed Changes

Browser tab:

Sidebar:

Footer:

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added, changed, renamed, or removed an environment variable, I have updated the list of environment variables in the documentation and made the necessary changes to the validator script according to the guide

[gabmontes]

This PR does not need to be merged. However, the assets used by the backed and set in hemilabs/blockscout#12 reference commits in this PR.

Once the assets are moved to a "branding" page, the environment variables shall be updated to point to those assets instead.

[gabmontes]

The logos are available at the Branding page, but the URLs do not seem to be stable enough to link to them from here.

A hemi-icons repository should be created or a more stable place should be defined to store those files.