resolvePage and resolvePageURL helpers should accept 0 arguments

If the resolvePage and resolvePageURL UI helpers are called with no arguments (only options), they should resolve the current page (the page from which they are called). Currently, resolving the current page requires passing the page.relativeSrcPath variable.

{{#with (resolvePage page.relativeSrcPath)}}
{{/with}}

It should be possible to simplify this call as follows:

{{#with (resolvePage)}}
{{/with}}

To access the raw page object (not the UI model for it), it should be possible to simplify this call:

{{#with (resolvePage page.relativeSrcPath model=false)}}
{{/with}}

to this one:

{{#with (resolvePage model=false)}}
{{/with}}

The same change should be made for resolvePageURL for completeness, though it would only function as a longer way to express page.url.

Note that an even simpler way to access the page's file is the proposed page.file property (see #1165 (closed)).