Make Feature calls use symbols consistently

We currently use both symbols and strings e.g. Feature.enabled?(:flag) and Feature.enabled?('another_flag')

Proposal

Standardise on symbols

Data

Count :

tkgl:gitlab tkuah$ gg Feature.enabled\?\(: |wc
       7      25     740
tkgl:gitlab tkuah$ gg Feature.enabled\?\(\' |wc
       9      35     902
tkgl:gitlab tkuah$ gg Feature.enabled\?\(\" |wc
       0       0       0