Change form buttons to use same css (and therefore match) login button

[maebeale]

The following forms are set up using button-primary
health_attributes/_form
projects/_form
targets/_form
tasks/_form

The button style we'd like to use is currently applied on sessions/new.html.erb via 'login-button' class.

We'd like for this to instead be an application-wide class applied to these forms (and sessions/new) so all action buttons look the same.

[vvtrip]

Please review my PR for this issue