add leading slash to page.url and post.url

Liquid:
```
{% for page in site.pages %}
    {{ page.url }}
{% endfor %}
```
Example output:
```
/docs/api/development/items.html
index.html
sitemap.txt
/blog/index.html
```
This sometimes-present leading slash makes it hard to reliably combine that URL, e.g. with `site.url`. I had to use the following dirty hack:
```
{% for page in site.pages %}
    {{ site.url }}{% capture first_letter %}{{ page.url | truncate: 1, '' }}{% endcapture %}{% unless first_letter == '/' %}/{% endunless %}{{ page.url }}
{% endfor %}
```

I would expect it to be consistent, no matter if that means always or never a leading slash. In this PR however, I removed it (can of course change it if you want me to).