Move shared HTML templates to shared-templates folder and convert apps to use symlinks

- Created shared-templates folder with common, postgres, and mongo subfolders
- Converted all sample apps to use symlinks pointing to shared templates
- Postgres apps (flask-postgres, flask-postgres-gevent, flask-postgres-xml, starlette-postgres-uvicorn, quart-postgres-uvicorn, fastapi-postgres-uvicorn) now use postgres shared templates
- MySQL/MSSQL/Clickhouse apps (flask-mysql, flask-mysql-uwsgi, flask-mssql, flask-clickhouse-uwsgi) now use common shared templates
- Mongo apps (flask-mongo, quart-mongo) now use mongo shared templates
- App-specific templates (flask-mysql extra templates, flask-postgres-xml/xml.html) left unchanged

This reduces duplication and makes it easier to maintain consistent templates across all sample applications.

Author: bitterpanda63

sample-apps/fastapi-postgres-uvicorn/templates/create_dog.html

@@ -0,0 +1 @@
+../../shared-templates/postgres/create_dog.html

sample-apps/fastapi-postgres-uvicorn/templates/create_dog.html

@@ -0,0 +1 @@
+../../shared-templates/postgres/dogpage.html

sample-apps/fastapi-postgres-uvicorn/templates/dogpage.html

@@ -0,0 +1 @@
+../../shared-templates/postgres/index.html

sample-apps/fastapi-postgres-uvicorn/templates/dogpage.html

@@ -0,0 +1 @@
+../../shared-templates/common/create_dog.html

sample-apps/fastapi-postgres-uvicorn/templates/index.html

@@ -0,0 +1 @@
+../../shared-templates/common/dogpage.html