Fix check to see if cart uses only DAI

[mds1]

Closes #7056

During checkout, if the user selects DAI for each grant contribution in their cart, a special heuristic is used to estimate gas. Before this PR, this heuristic would be accidentally applied any time a cart used the same token for each grant in their cart, regardless of whether or not that token was DAI.

This PR resolves that so the gas estimation heuristic is now only used for carts with DAI

cc @owocki

[codecov]

Codecov Report

Merging #7079 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7079      +/-   ##
==========================================
+ Coverage   26.25%   26.27%   +0.02%     
==========================================
  Files         300      300              
  Lines       29323    29323              
  Branches     4318     4318              
==========================================
+ Hits         7699     7705       +6     
+ Misses      21353    21347       -6     
  Partials      271      271              

Impacted Files	Coverage Δ
app/dashboard/embed.py	31.60% <0.00%> (+3.44%)	⬆️

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