Update frequency response based on user feedback

[danich1]

This PR is in response to #60. The quick change here is that I added a field for frequency to have a normalized form based on all tokens found within a given year. Backend response now has a field called normalized frequency.

Main frontend changes will need to be as follows below (I think it should be a very quick fix🤞🏾) @vincerubinetti:

• the frequency visualization will need a feature to allow users to toggle between normalized frequency and regular frequency
• the frequency visualization will need a feature that allows users to change from linear to log scale when displaying the regular frequency. (normalized version will only need to be reported in scientific notation e.g. 2.5e-3)

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