html: replaced slack image with font-awesome icon

[thelostone-mc]

Description

replaced img with font-awesome -> page loads a tad bit faster
If this is alright, will do it in batches so as to use icons rather than images + refactor along the way

• replaced slack image with slack font-awesome
• re-indented nav
• noticed that nav bars have different size on different pages, so evened it out

Checklist

• linter status: 100% pass
• changes don't break existing behavior
• commit message follows commit guidelines

Affected core subsystem(s)

• html

[codecov]

Codecov Report

Merging #333 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #333      +/-   ##
==========================================
- Coverage   23.56%   23.54%   -0.02%     
==========================================
  Files          82       82              
  Lines        4083     4082       -1     
  Branches      491      491              
==========================================
- Hits          962      961       -1     
  Misses       3113     3113              
  Partials        8        8

Impacted Files	Coverage Δ
app/dashboard/models.py	39.17% <0%> (-0.15%)	⬇️

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[owocki]

as youre submitting these frontend PRs it might be nice to see screenshots of your work without having to pull the branch :)

[thelostone-mc]

@owocki Yeah I was getting to that 😅

[thelostone-mc]

Most pages in gitcoin have a navbar of this size:

But pages like the landing, help have a larger nav bar

After fix:

[thelostone-mc]

@owocki review + merge ?

[owocki]

thanks!