feat(admin): Allow % in extra part of system query

[lynnagara]

Support queries like 'WHERE some_column LIKE '%something%'

[codecov-commenter]

Codecov Report

Merging #2320 (74faa11) into master (4483262) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2320   +/-   ##
=======================================
  Coverage   92.79%   92.79%           
=======================================
  Files         558      558           
  Lines       25642    25642           
=======================================
  Hits        23794    23794           
  Misses       1848     1848           

Impacted Files	Coverage Δ
snuba/admin/clickhouse/system_queries.py	68.81% <ø> (ø)

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