Don't serialize back to Pinboard API named fields in template

[JeanMertz]

The Pinboard API calls a title description, and a description extended. When deserializing the JSON response to the Post struct, we rename those fields using Serde:

bulletin/src/main.rs

Lines 52 to 55 in e16f06e

	#[serde(rename = "description")]
	title: String,
	#[serde(rename = "extended")]
	description: String,

However, when serializing the Posts to be used in the final template (using Tera's built-in support for Serde), the fields get renamed back to their original Pinboard-described names:

bulletin/src/main.rs

Lines 109 to 114 in e16f06e

	fn render(template: &str, data: Data) -> Result<String, String> {
	match Tera::one_off_value(template, &data, true) {
	Ok(s) => Ok(s),
	Err(err) => Err(format!("{}", err)),
	}
	}

We should keep the new title and description field names instead. This can be done using the (undocumented) attribute:

#[serde(rename(deserialize = "description"))]