feat: Implement mailer service for sending emails

src/service/templates/mail/_base.txt.j2

@@ -0,0 +1,3 @@
+{%- block content %}{% endblock %}
+
+Message sent by Continuwuity {{ env!("CARGO_PKG_VERSION") }}. 🐈

src/service/templates/mail/change_email.txt.j2

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+{% extends "_base.txt.j2" %}
+
+{% block content -%}
+Hello!
+
+Somebody, probably you, tried to associate this email address with the Matrix account {{ user_id }}.
+If that's your account, and this is your email address, click this link to proceed:
+    {{ verification_link }}
+Otherwise, you can ignore this email. The above link will expire in one hour.
+{%- endblock %}

src/service/templates/mail/new_account.txt.j2

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+{% extends "_base.txt.j2" %}
+
+{% block content -%}
+Hello!
+
+Somebody, probably you, tried to create a Matrix account on {{ server_name }} using this email address.
+Use the link below to proceed with creating your account:
+    {{ verification_link }}
+If you are not trying to create an account, you can ignore this email. The above link will expire in one hour.
+{%- endblock %}

src/service/templates/mail/password_reset.txt.j2

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+{% extends "_base.txt.j2" %}
+
+{% block content -%}
+Hello {{ display_name }} ({{ user_id }}),
+
+Somebody, probably you, tried to reset your Matrix account's password.
+If you requested for your password to be reset, click this link to proceed:
+    {{ verification_link }}
+Otherwise, you can ignore this email. The above link will expire in one hour.
+{%- endblock %}

src/service/templates/mail/test.txt.j2

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+{% extends "_base.txt.j2" %}
+
+{% block content -%}
+If you're seeing this, SMTP is configured correctly. :3
+{%- endblock %}