Fix renaming of ERB file names
Fix renaming of ERB file names

ERB/Rails requires partials to be named in `snake_case`. Using
`str.replace` with string match value will only replace the first
instance of a match. Using a `/g` regex will match and replace all.

alphagov/govuk_frontend_alpha,alphagov/govuk_frontend_alpha,alphagov/govuk_frontend_alpha