Fix naming of files for case-insensitive file systems

[asymetrixs]

Cloning the project leads to the following message on MacOSX, path src/Cofoundry.Samples.SPASite.Domain/MailTemplates

# git clone https://github.com/cofoundry-cms/Cofoundry.Samples.SPASite.git
Cloning into 'Cofoundry.Samples.SPASite'...
remote: Enumerating objects: 2672, done.
remote: Counting objects: 100% (348/348), done.
remote: Compressing objects: 100% (204/204), done.
remote: Total 2672 (delta 174), reused 257 (delta 135), pack-reused 2324
Receiving objects: 100% (2672/2672), 25.01 MiB | 18.65 MiB/s, done.
Resolving deltas: 100% (1444/1444), done.
warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'src/Cofoundry.Samples.SPASite.Domain/MailTemplates/NewUserWelcomeMail_Html.cshtml'
  'src/Cofoundry.Samples.SPASite.Domain/MailTemplates/NewUserWelcomeMail_html.cshtml'
  'src/Cofoundry.Samples.SPASite.Domain/MailTemplates/NewUserWelcomeMail_Text.cshtml'
  'src/Cofoundry.Samples.SPASite.Domain/MailTemplates/NewUserWelcomeMail_text.cshtml'