Review settings forms concerning browser validation attributes

[mstilkerich]

Revisit the UI settings forms for accounts and addressbooks and check where we can user browser builtin validation to improve usability, e.g.:

• Specific input types may be available for some input (e.g., URL), which may allow browsers to do things as showing more appropriate keyboards on mobile devices
• Mark required form fields as such so the browser can check on submission they have actually been filled
• Placeholder text may be useful for some fields to indicate expected format
• Title attribute could provide useful help, especially concerning available placeholders
• Regex pattern can be specified for fields such as refresh time to enable the browser to validate the input

[mstilkerich]

• More specific input types not found. URL is not usable for the discovery URL as at least Safari expects a full URL including scheme, whereas we also want to accept a simple domain name here. For the time intervals, the time pickers are not usable because they allow to pick a time of day only (i.e. limited to 24 hours max)

• Required form fields have been marked as such. This includes the name fields; although the backend does not care about empty account or addressbook names, from a user point of view it makes little sense to leave them empty.

• Placeholder text has been added to indicate format of the time strings.

• Regex patterns check that the time strings and domain name are in the expected format (the domain name pattern is very minimalistic and does not really check the validity of a URL).