Support directory installations

- Add web log to HttpContext on first retrieval per req
- Add several DotLiquid filters
- Use int for page numbers
- Update all themes to use rel/abs link and other filters

src/MyWebLog/themes/default/index.liquid

@@ -7,7 +7,7 @@
       <div class="col">
         <article>
           <h1>
-            <a href="/{{ post.permalink }}" title="Permanent link to &quot;{{ post.title | escape }}&quot;">
+            <a href="{{ post | relative_link }}" title="Permanent link to &quot;{{ post.title | escape }}&quot;">
               {{ post.title }}
             </a>
           </h1>

src/MyWebLog/themes/default/layout.liquid

@@ -13,7 +13,7 @@
     <header>
       <nav class="navbar navbar-light bg-light navbar-expand-md justify-content-start px-2">
         <div class="container-fluid">
-          <a class="navbar-brand" href="/">{{ web_log.name }}</a>
+          <a class="navbar-brand" href="{{ "" | relative_link }}">{{ web_log.name }}</a>
           <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarText"
                   aria-controls="navbarText" aria-expanded="false" aria-label="Toggle navigation">
             <span class="navbar-toggler-icon"></span>